APM has an companion OSD board called MinimOSD. It reads all the MAVLink data in the APM telemetry stream and overlays it on the video stream if you're using an on-board camera and wireless video transmitter. This is useful if you're flying in First Person View (FPV) mode or don't want to use a laptop at the field to see your telemetry data in the Mission Planner.
Instructions on connecting and using the MinimOSD board via the APM telemetry port are here. Connection photos with APM 1 and APM 2 are below.